Question

what are the coefficients, reading left to right, when the chemical equation below is balanced?
? pcl₃(l) + ? h₂o (l) → ? h₃po₃ (aq) + ? hcl (g)
drag the oval over your answer.
1, 3, 2, 1 1, 2, 1, 1
2, 6, 1, 9 1, 3, 1, 3

Explanation:

Step1: Balance phosphorus atoms

There is 1 P on each side, so coefficient of $\text{PCl}_3$ and $\text{H}_3\text{PO}_3$ is 1 initially:
$1\text{ PCl}_3(l) + ?\text{ H}_2\text{O}(l)
ightarrow 1\text{ H}_3\text{PO}_3(aq) + ?\text{ HCl}(g)$

Step2: Balance chlorine atoms

3 Cl on left, so coefficient of $\text{HCl}$ is 3:
$1\text{ PCl}_3(l) + ?\text{ H}_2\text{O}(l)
ightarrow 1\text{ H}_3\text{PO}_3(aq) + 3\text{ HCl}(g)$

Step3: Balance hydrogen atoms

Total H on right: $3 + 3 = 6$. So $\text{H}_2\text{O}$ needs coefficient 3 ($3\times2=6$):
$1\text{ PCl}_3(l) + 3\text{ H}_2\text{O}(l)
ightarrow 1\text{ H}_3\text{PO}_3(aq) + 3\text{ HCl}(g)$

Step4: Balance oxygen atoms

O on left: $3\times1=3$, O on right: $3$. Equation is balanced.

Answer:

1, 3, 1, 3